Defects in the regulation of intracellular Ca2+ released from sarcoplasmic reticulum (SR) stores have been demonstrated in inherited arrhythmia syndromes such as catecholaminergic polymorphic ventricular tachycardia (CPVT), but may also provide an arrhythmogenic trigger in acquired arrhythmias like atrial fibrillation (AF). Recent studies have revealed defects in the regulation of ryanodine receptor (RyR2) Ca2+ release channels in patients with AF. AF is the most prevalent arrhythmia, affecting more than 2 million Americans each year and causing a twofold increase in mortality. Since the activity of RyR2 is strongly regulated by Ca2+/calmodulin-dependent kinase (CaMKII), which binds to RyR2 and is activated in response to faster heart rates, it has been proposed that abnormal CaMKII regulation plays/a role in the onset of AF. The long-term goal of this project is to define the cellular/ molecular mechanisms by which abnormal CaMKII phosphorylation of RyR2 at serine 2814 (S2814) leads to AF, by studying human atrial biopsies and genetically-altered mice in which RyR2 activity has been altered, or the CaMKII phosphorylation site on RyR2 has been inactivated. Our hypothesis is that direct binding of CaMKII to RyR2 enables the enzyme to sense and amplify SR Ca2+ leak, which may increase the likelihood of AF in hearts more susceptible to arrhythmias due to an inherited mutation in RyR2 (R176Q) or the absence of the RyR2- stabilizing subunit FKBP12.6. The specific aims are to: 1) Demonstrate that CaMKII phosphorylation at S2814 of RyR2 is increased in human atrial biopsies from patients with AF;2) Define the molecular mechanisms by which CaMKII regulates RyR2;3) Determine if CaMKII phosphorylation of RyR2 increases the probability of AF in RyR2-R176Q mutant or FKBP12.6-deficient mice;4) Determine if prevention of CaMKII phosphorylation of RyR2 in FKBP12.6-deficient mice decreases susceptibility to AF. We propose to conduct translational studies ranging from single channel measurements of RyR2, Ca2+ imaging in isolated cardiomyocytes and arrhythmia-inducibility studies in genetically-altered mice, to elucidate the molecular mechanisms underlying RyR2-dependent initiation of AF. It is anticipated that the results of these studies will advance our understanding of CaMKII-dependent mechanisms underlying the initiation of AF as well as other cardiac arrhythmias associated with abnormal Ca2+ homeostasis. Furthermore, the reagents and animal models developed for this project may be utilized to create new drugs for AF and other common diseases of the heart.